1. Field
The present inventive concept relates to a method of preparing polylactic acid, a polylactic acid resin prepared using the method, a resin composition including the polylactic acid resin, and a catalyst system for preparing polylactic acid.
2. Description of the Related Art
In view of environmental protection, there is increasing attention given to biodegradable resins such as aliphatic polyester. As a biodegradable resin, polylactic acid (or polylactide) has a high melting point of about 130° C. to about 180° C. and has good transparency. Lactic acid as a source material of polylactic acid may be obtained from renewable resources such as plants.
In general, polylactic acid may be synthesized by direct condensation polymerization of lactic acid or ring opening polymerization of lactide. The direct condensation polymerization of lactic acid is an equilibrium reaction. Thus, it is practically difficult to obtain a polylactic acid having a high molecular weight of about 100,000 or greater without removing the water produced during the equilibrium reaction under high temperature or reduced pressure. However, the ring opening polymerization of lactide does not generate a byproduct, and thus is suitable for preparing a high molecular weight polylactic acid.
In the ring opening polymerization of polylactic acid, a variety of catalysts may be used. For example, an organotin compound may be used as a catalyst. However, organotin compounds may cause a side reaction such as a chain transfer reaction during the ring opening polymerization. Thus it is difficult to prepare a high molecular weight polylactic acid with a high yield in a short period of time.
Therefore, there is still a need for a method of preparing a high molecular weight polylactic acid with a high yield in a short period of time.